1. Field of the Invention
This invention relates to a honeycomb structure and a method for manufacturing a honeycomb structure.
2. Discussion of the Background
In recent years, particulate matter (hereinafter, also referred to as PM) such as soot contained in exhaust gases discharged from internal combustion engines of vehicles such as buses and trucks, construction machines and the like, and other harmful contaminants have raised serious problems to the environment and the human body.
For this reason, various porous ceramic honeycomb structures have been proposed as filters that capture PM in exhaust gases and purify the exhaust gases and also as catalyst supporting carriers that convert contaminants such as CO and HC in exhaust gases by allowing the exhaust gases to pass through the inside of the catalyst supporting carriers.
With respect to such a honeycomb structure, there has been known an aggregated honeycomb structure formed by combining a plurality of honeycomb fired bodies each having a large number of cells longitudinally placed in parallel with one another. In such an aggregated honeycomb structure, an adhesive layer is formed on each of side faces of the honeycomb fired bodies, and the respective honeycomb fired bodies are bonded to one another with the adhesive layer interposed therebetween.
Upon use of a honeycomb structure for purifying or converting exhaust gases, the honeycomb structure allows high-temperature exhaust gases to flow through cells of the honeycomb structure. Consequently, thermal shock is applied to the honeycomb structure during use, and when the thermal shock is increased, the honeycomb structure tends to be damaged.
However, by combining honeycomb fired bodies with one another with an adhesive layer interposed therebetween to form an aggregated honeycomb structure, the thermal shock applied to the honeycomb structure is presumably alleviated by the adhesive layer so that it is possible to prevent the damage in the honeycomb structure.
WO 01/23069 A1 has described, as an aggregated honeycomb structure, a honeycomb structure formed as follows: using rectangular pillar-shaped honeycomb fired bodies having a square shape in its cross-section taken perpendicular to the longitudinal direction thereof; integrating a plurality of the honeycomb fired bodies by bonding with an adhesive layer containing an adhesive interposed therebetween; and cutting a periphery into a round pillar-shape.
The adhesive layer in the honeycomb structure described in WO 01/23069 A1 contains inorganic fibers, an inorganic binder, an organic binder and inorganic particles, and its thermal conductivity is 0.1 to 10 W/mK. Moreover, all the honeycomb fired bodies are bonded to one another with the adhesive layer having the same composition.
Here, in processes for manufacturing a honeycomb structure described in WO 01/23069 A1, honeycomb fired bodies located at four corners of the rectangular pillar-shaped integral body of the honeycomb fired bodies prior to the periphery cutting are mostly cut off in the periphery cutting. As a result, the cross-sectional areas of the respective honeycomb fired bodies located at the four corners are smaller than those of the honeycomb fired bodies located in a center portion in the cross-section of the honeycomb structure (hereinafter, referred to as center-portion honeycomb fired bodies).
The contents of WO 01/23069 A1 are incorporated herein by reference in their entirety.